The present invention relates to optical fiber waveguides. More particularly, it concerns a method for fabricating optical fiber preforms from which optical fibers can be drawn and apparatus for accomplishing the method.
Optical fibers are typically fabricated from a high-purity silica and have a light transmitting core surrounded by a cladding layer. The indices of refraction of the core and the cladding are adjusted during manufacture to provide the cladding with an index of refraction that is less than that of the core to enable guided light transmission in the core. The finished fiber is typically drawn from a silica starting preform that has the same cross-sectional geometry of the finished fiber, but a diameter several orders of magnitude greater than that of the fiber. In drawing a fiber, one end of the preform is heated in a furnace and the fiber drawn in one or more drawing steps.
Various methods and variations thereof have been developed for fabricating preforms. In one method, known as the outside vapor deposition (OVD) process, silicon tetrachloride, oxygen, and hydrogen are combined in a burner nozzle and reacted in a flame at a temperature between 1400.degree. and 1800.degree. C. The hydrogen functions as a fuel with the silicon tetrachloride and the oxygen reacting to form high purity silica particles. The index of refraction of the resulting silica can be controlled by adding various dopants to the starting gases, for example, boron or fluorine compounds can be added to effect a lowering of the index of refraction while compounds of phosphorous or germanium can be added to effect an increase in the index of refraction.
The silica particles formed as a consequence of the reaction are deposited onto a suitable substrate that often times takes the form of an elongated cylindrical rod having a nominal starting diameter (e.g., &lt;12 mm). In order to form a uniform deposit of the silica particles, it is common to rotate the substrate as well as move the depositing flame and the substrate axially relative to one another. A more sophisticated variation of this process employs a circular burner, having a diameter between two and four inches, with concentric rings of holes or slots through which the reactant gases flow to provide a generally cylindrical stream of silica particles. When a circular burner is used with a nominal diameter cylindrical starting rod, the initially presented surface area of the rod represents a relatively small target upon which the silica particles can be deposited. As a result, the deposition rate during the initial stage of preform fabrication is quite low (e.g., &lt;0.5 gm/min) with a relatively large portion of the silica particles being formed by the circular burner not reaching their intended target surface. As the preform slowly increases in size with continued deposition, the surface area presented for receiving the silica particles increases to permit an increase in the deposition rate. With increased preform growth allowing for higher deposition rates, the deposition process accelerates until the desired preform diameter is attained, at which time the deposition rate can be several grams/minute. Since the burner must be large enough in diameter to accommodate the gas flow rates, the slow initial deposition rates can be regarded as a necessary, though not particularly efficient, incident to the use of circular burners. As can be appreciated, a need arises for overcoming the initial slow deposition rates associated with the use of circular burners as well as generally increasing the overall deposition rate.